Molecular dynamics study of confined water in the periclase-brucite system under conditions of reaction-induced fracturing

MG Guren and HA Sveinsson and A Hafreager and B Jamtveit and A Malthe- Sorenssen and F Renard, GEOCHIMICA ET COSMOCHIMICA ACTA, 294, 13-27 (2021).

DOI: 10.1016/j.gca.2020.11.016

The volume-increase associated with hydration reactions in rocks may lead to reaction-induced fracturing, but requires a stable water film to be present at reactive grain boundaries even when subject to compressive stress. Hydration of periclase to brucite is associated with a solid volume increase of ca. 110%. Recent experiments on the periclase-brucite system observed that when the effective mean stress exceeds 30 MPa, the reaction rate slows down dramatically. We hypothesize that for the brucite forming reaction to progress, the fluid film between grains must remain stable. If the applied pressure becomes larger than the hydration force, the fluid film will collapse and be squeezed out of the grain contacts. To quantify this effect, we study the behavior of a water film confined between periclase or brucite surfaces subject to compressive stress, by performing molecular dynamics simulations. The simulations are carried out using the ClayFF force field and the single point charge (SPC) water model in the molecular dynamics simulations program LAMMPS. The setup consists of two interfaces of either periclase or brucite surrounded by water. Our simulations show that when the pressure reaches a few tens of MPa, the water film collapses and reduces the water film to one or two water layers, while the self-diffusion coefficient of water molecules by a factor of eight. A water film thickness below two water layers is thinner than the size of the hydration shell around Mg2+ -ions, which will limit ion-transport. The observed collapse of the water film to a single layer at a normal pressure of 25-30 MPa might explain the observed slow-down of reaction-induced fracturing in the periclase-brucite system. (C) 2020 The Author(s). Published by Elsevier Ltd.

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